This invention relates to a piezoelectrically operated apparatus which is useful for switching electrical circuits. More particularly, it relates to piezoelectric circuit elements which are specially constructed to have a combination of increased separation between the switching contacts when the contacts are in the open position and increased closing force when the contacts are in the closed position.
Electromagnetic relays have been used in the past to switch a wide range of electrical circuits by separating or closing one or more pairs of electrical contacts. While electromagnetic relays perform satisfactorily for some applications, they can be slow acting, relatively large in size, and costly. They typically require a relatively bulky solenoid coil and associated linkage to provide contact movement. Such coil and linkage systems are, in addition to being a major part of the relay cost, generally energy inefficient. Furthermore, electromagnetic relays do not lend themselves to synchronous operation. Although conventional electromagnetic relays may be employed to switch such loads as, for example, an alternating current electrical circuit on demand, the movement of the contacts is usually random on the time scale of the load current waveform, because of generally long mechanical reaction times associated with the operation of such relays. As a result, opening and closing operations of the contacts are not synchronized with the zero current points of the current waveform, especially when the load is an alaternating current circuit. For electrical circuits operating at current levels typical of such power sources as household electrical wiring, opening and closing of the relay contacts is often accompanied by arcing between the contacts. When the current in such a circuit is interrupted, the current through the relay contacts does not drop to zero at the instant of contact separation, but rather persists in the form of an arc between the contacts, usually until the alternating current waveform approaches the next sinusoidal zero. As the current level decreases toward the sinusoidal zero point, the art becomes unstable and is suddenly extinguished, a phenomenon often referred to as chopping. This sudden extinction at low current represents an extremely high rate of change of current. As a result, if the electrical circuit in which the current is being interrupted has significant inductance, high voltage transients, proportional to the product of the inductance and the rate of change of the current, are produced. These voltage transients may cause electrical breakdown in either the equipment connected with the circuit, or the relay itself, or both. Moreover, such arcing is damaging to the contacts themselves and can cause contact erosion and contact welding. It is therefore desirable to minimize any arcing occurring between the relay contacts when the contacts are opened or closed. One way to minimize such arcing is to operate the relay so as to switch the electrical circuit at a point in the load current waveform where the current level is as close to zero as possible, which operation is referred to hereinafter as synchronous operation.
A piezoelectric device, utilizing the fast action capability of a piezoelectric bender, may be employed to provide a synchronously operable switching relay. Synchronous operation requires that the relay contacts be moved between the open and closed positions in a relatively short period of time. The fast action, relatively low mass, and small travel distance of a piezoelectric bender facilitate the use of such a device in a synchrounously operable relay. A further characteristic of a piezoelectric device is that the deflecting force acting to move the contacts is at a maximum at the beginning of the piezoelectric bender's deflection. This characteristic further enhances the device's capability of moving the relay contacts in a short period of time. With this fast action capability, a piezoelectric relay may be operated so that the contacts are opened or closed at a time very close to the time when the current level is zero in the circuit being switched, thereby substantially reducing contact erosion, contact welding, and transient inductive voltages. Also, the simplicity of a piezoelectric device avoids most of the mechanical problems of conventional electromagnetic relays, and the energy efficiency of such a device permits operation with far less expenditure of energy.
However, the displacement achievable for piezoelectric benders of small size is relatively small, especially if the bender is operated in a so-called "inchworm" configuration where the two ends of the bender remain fixed while the middle deflects in response to an applied electrical signal. For example, for a bender which is useful in a synchronously operable switching apparatus, the deflection achievable may be as little as 10-20 mils. For some applications, such small deflections may not provide sufficient contact separation to meet the dielectric strength requirements of the application involved. Furthermore, the contact closing force available from a piezoelectric bender is also relatively small, and the available force is at a minimum at the end of the bender's deflection. While the force characteristic of a piezoelectric bender, which is at a maximum at the beginning of the bender's travel and at a minimum at the end thereof, is useful for synchronously operating relay contacts, in that such force characteristic gives fast action capability, it also may result in lower residual forces in the closed position of the relay contacts, than is characteristic of an electromagnetic relay. If the residual closing force on the contacts is too small, the electrical resistance of the closed contact interface, conventionally referred to as contact resistance, for a particular circuit may be unacceptably high. High contact resistance results in excessive power loss at the contact interface, and the resistive heating at the interface can lead to a number of contact failure modes. For these reasons, the full displacement achievable by a piezoelectric bender is not available when the bender is employed in a switching apparatus. Instead, only a portion of the achievable displacement is used to provide contact separation, with the remainder being used to provide a residual closing force when the contacts are in the closed position.
Piezoelectric benders have been used in the past in a number of applications, including utilization in various piezoelectric relays. For example, piezoelectric benders used as relay elements are described in U.S. Pat. Nos. 2,166,763, 2,182,340, 2,471,967, 2,835,761, 4,093,883, and 4,403,166. However, none of the piezoelectric relays disclosed by these patents have been specifically designed to minimize arcing. No consideration has been given to providing a synchronously operable relay, or to one which is especially useful for switching electrical circuits operating at household power line current levels. As has been noted above, switching circuits operating at such current levels results in significant arcing if the circuit is not switched at a point in time close to a sinusoidal zero of the alternating current level. Application Ser. No. 684,881, assigned to the same assignee as the present invention and filed concurrently herewith, discloses a piezoelectric relay having a very small gap length as compared to the contact separation for conventional relays, which relay may be synchronously operated so that, for example, 110 volt alternating current circuits are switched on and off with minimal arcing between the relay contacts. Application Ser. No. 684,882, also assigned to the present assignee and filed concurrently herewith, discloses a synchronously operable electrical current switchng apparatus which employs a plurality of piezoelectric benders and which may be used to switch multiple circuits or to lower the contact resistance in one or a few circuits. The present invention provides a synchronously operable electrical current switching apparatus which provides increased separation between the switching contacts when the contacts are in the open position and increased closing force when the contacts are in the closed position.
It is seen from the above that it is an object of the present invention to provide a piezoelectrically actuated electrical current switching apparatus that exhibits increased contact separation when the contacts are in the open position and increased closing force when the contacts are in the closed position.
It is another object of the present invention to provide an electrical current switching apparatus which is fast acting, small in size, highly energy efficient, and low in cost.
It is also an object of the present invention to provide an electrical circuit switching apparatus which is synchronously operable.